We propose to pursue two related biologic questions that grew out of studies we performed in the current funding period of this grant. These questions deal with the expression of cyclooxygenase-2 (Cox-2) in intestinal epithelial cells and the effects of the prostaglandins produced by Cox-2 on the biologic response of intestinal epithelial cells to radiation injury. These biologic questions are: 1). What is the mechanism of the constitutive regulation of Cox-2 by lymphotoxin? Cox-2 is expressed transiently in epithelial cells in response to cytokines and growth factors but is expressed constitutively in colon cancer and inflammatory bowel disease. The intracellular signaling events regulating transient Cox-2 expression are well defined but the events regulating constitutive expression are not. We have identified a system, the binding of lymphotoxin (LT) to the LTBeta receptor, that regulates constitutive Cox-2 expression. Our first hypothesis is that binding of LTalpha1Beta2 to the LTBeta receptor prevents constitutive Cox-2 expression and interruption of this system results in constitutive Cox-2 expression. Our first Specific Aim is to define the molecular events in the regulation of constitutive Cox-2 expression by LT. 2). What intracellular signaling events mediate the biologic effects of PGE2 in irradiated intestinal epithelial cells? During the current funding period we demonstrated that the epithelial response to radiation injury including radioprotection, apoptosis and the stem cell survival is regulated by prostaglandins. We have developed data suggesting that the effects of PGE2 on these events in mice are mediated through binding to the EP2 receptor. Our second hypothesis is that in irradiated intestinal epithelial cells the biologic response to PGE2 (apoptosis, radioprotection and increased survival after radiation) are all mediated through binding to the EP2 receptor. Our second Specific Aim is to define the intracellular signaling events through which PGE2 affects apoptosis and the number of surviving cells after radiation in HCT-116 cells. These proposed studies should lead to a better understanding of the regulation of constitutive Cox-2 expression including constitutive Cox-2 expression in colon cancer. The ability to manipulate constitutive Cox-2 expression in colon cancer could have implications for therapy. These proposed studies should also give a better understanding of the epithelial response to radiation injury. Manipulation of the epithelial response to radiation could lead to agents that protect normal epithelial cells during radiation therapy.